In the art of container coating it is well known to coat both the interior and exterior surface areas of a metal can made of aluminum, steel or tin plate steel for example. Known coating materials and methods have included the spray coating of containers with conventional paints and electrophoretic coating of containers with such coatings as water-based or water dispersible resinous coating materials. This invention concerns apparatus for the electrodeposition of such materials on electrically conductive surface areas of a metal can from either anodic or cathodic electrocoating material or media.
Typically in the electrocoating art, the metal can is confined in a cell which is then filled with a continuous flow of coating material, and a layer of particulate coating material is then electro-deposited on the electrically charged metal surfaces of the can as a result of an electrical potential difference maintained between the can and the oppositely charged electrodes. The prior art includes among numerous examples of electrocoating technology, U.S. Pat. Nos. 3,476,667; 3,922,213; 4,094,760 and 4,210,507.
Inasmuch as electrocoating of containers as discussed hereinabove and disclosed in the cited prior art patents is well known, further detailed description thereof is not necessary for an understanding of the present invention and therefore is included hereinafter only as necessary for describing the invention. Such known electrocoating apparatus often has comprised rotary turret type machines that are employed for handling metal containers for the cleansing and coating thereof during can manufacture. Examples of rotary turret machines generally, for use in handling of containers for processing and for packaging of products therein include U.S. Pat. Nos. 3,476,666; 4,026,311; 4,158,405 and 4,246,088, and British Pat. No. 1,571,808 all disclose such apparatus U.S. Pat. Nos. 3,476,666 and 4,246,088 in particular disclose the use of rotary turret type apparatus in the processing of metal cans which includes such electrically operative treatment of cans as electrolytic surface treatment and electrodeposition of protective coating materials thereon.
In many prior rotary turret type machines for electrocoating of containers, each of the containers to be coated is carried in one of the plurality of cells which are positioned about a pitch circle that is coaxial with the axis of rotation of the turret machine. Accordingly, a loading station and a discharge station must also be provided, as well as other ancillary container handling apparatus well known in the art.
Conventional rotary turret machines typically have cells comprised of upper and lower vertically movable cell portions, and an intervening can support portion which supports a can in an inverted orientation. The difficulty of providing all of the requisite electrical and fluid connections to such cells for the electrocoating process significantly impacts the complexity of the apparatus, and therefore its cost, without any compensating benefit. Each such cell of the rotary turret machine must be provided with fluid supply connections to supply a flow of electrocoating medium to all areas of the can's surface, usually including both interior and exterior surface areas, and an exhaust channel to maintain the flow of coating material during the coating cycle and to drain away residual coating material at the end of the coating cycle. Additionally, motive means such as a mechanical rise and fall cam must be provided to open and close each vertically moving portion of each cell as required during the machine operating cycle. Still further, electrical connections for the impression of an electrical potential difference between the container to be coated and the oppositely charged electrodes must be provided for each vertically movable portion of the cell.
As a rotary turret type machine requires phased operation of the individual cells according to the position of each cell on the circumference of the turret at any given time, all of the cell cycle timing and the necessary control of the electrocoating material supply and exhaust, electrical connections, cell opening and closing mechanisms, and all of the other operative entities required in the electrocoating process must be similarly phased to provide the proper cyclic operation for each cell. This requirement has resulted in enormously complex apparatus.
The more complex a machine is, generally the more expensive and less reliable it will be than a simpler design of comparable capability. The phased relationship of the cell operating cycles in rotary turret type electrocoating machines introduces considerable complexity of mechanical design and control as the cells operate neither in unison or in a distinct sequence of repetitive non-overlapping cycles. For the above and other reasons, known rotary turret type electrocoating apparatus has not proven to be entirely satisfactory, although its acceptance and use is nevertheless widespread.
Other related problems have been evident in the prior art. For example, gravity and the high speed rotation of rotary turret type machines tend to direct leakage of electrocoating material from the cell closure interface radially outward and downward where it may foul electrical commutators and brushes, the lower cell portion rise and fall cam, or other components located on or near the circumferential perimeter of the machine and below the interface between the upper and lower cell portions.
Apart from the above and other problems of prior rotary turret machines, prior electrocoating cells have been subject to certain limitations irrespective of the type of apparatus in which they are incorporated. For example, practitioners of the art have continuously sought to minimize coating cycle time, or in other words to maximize coated can production rates. The rate of coating deposition on the can is one of several critical factors that determine minimum cycle time. Coating deposition rate is in turn a function of several design and operating perameters including the percentage of resin in the coating bath, the applied electrical potential difference between the can and adjacent electrodes, and the can-to-electrode spacing. As a result, several critical design parameters of conventional electrocoating cells often have come into direct conflict and have impeded efforts to maximize production rates.
For example, it has been suggested that improvements in electrocoating cycle times might be achieved by placing the electrode in very close proximity to the can surface to be coated; however, this would narrow the coating medium flow channel between the can surface and the electrode and therefore reduce the cross-sectional flow area of the channel. The resulting restriction in coating medium flow would introduce a higher pressure drop through the flow channel and would therefore require more motive force to maintain the same level of coating medium flow. Furthermore, with such a flow channel cross-section reduction, the reservoir volume available for coating medium is reduced and the coating medium flow rate thus would have to be increased to offset the reduced availability of fresh coating medium within the cell. Failure to do this could well result in resin starvation in the coating medium and consequent reduced electrodeposition rates. A related concern is that significant reductions in flow channel cross-sectional area and increased electrocoating medium pressures of flow rates will tend to promote fluid turbulance in the coating medium flow and inhibit the coating process.
Thus it will be appreciated that maximized electrodeposition rates at minimum cost requires minimum effective electrical potential, minimum can-to-electrode spacing, elimination of coating medium flow turbulence, minimizing of coating medium flow channel restrictions, minimal effective flow pressures, and maintenance of a sufficient volume of fresh coating medium at all times. Due to the perceived interdependence of these conditions in the prior art, every prior cell design known to applicant has been at best a design compromise of limited success.